A pressure sensitive adhesive is generally a material which adheres to a surface with slight pressure and releases from the surface with negligible transfer of the adhesive to the surface. The most common examples are the adhesives used in bandages to cover wounds. Silicone pressure sensitive adhesives in particular have found use in transdermal drug delivery applications which involve the adherence of a drug-containing patch to a patient's skin, due to the fact that silicone pressure sensitive adhesives are acceptable for topical use.
In transdermal drug delivery patches, ingredients such as co-solvents and excipients have been added to the silicone pressure sensitive adhesive compositions to improve efficacy. Co-solvents are typically added to increase drug solubility in the composition, and excipients are typically added to enhance drug release from or through the composition.
However, when silicone pressure sensitive adhesives are formulated with or come in contact with co-solvents, excipients, drugs such as nicotine-based drugs, or skin penetration enhancers such as propylene glycolmonolaurate or glycerol monoleate, the silicone pressure sensitive adhesive often becomes plasticized, losing tack, adhesiveness, and resistance to flow. Such instances occur (1) in matrix-type drug delivery patches where a drug is formulated into a silicone pressure sensitive adhesive and (2) in reservoir-type drug delivery patches where a silicone pressure sensitive adhesive is on the surface of a drug delivery device containing a reservoir of a drug. In the former type of patch, the silicone pressure sensitive adhesive is in intimate contact with the drug and other possible plasticizing additives. In the latter type of patch, the silicone pressure sensitive adhesive is present to provide means for attaching the device to a patient's skin. In this device, drugs or other materials from the reservoir either pass through or otherwise may come in contact with the silicone pressure sensitive adhesive.
The problem with either device is that the additives cause reduction in cohesive strength. Cohesive strength causes the adhesive to adhere to a substrate such as a patch and prevents flow or transfer of the pressure sensitive adhesive onto either the release liner which protects the silicone pressure sensitive adhesive before use or onto the patient's skin following removal of the patch while maintaining adequate adhesion of the patch to the skin. The cohesive strengthening agent further prevents flow of the pressure sensitive adhesive beyond the edges of the patch during storage and/or use.
It is, therefore, a primary object of the invention to provide a pressure sensitive adhesive composition which has good cohesive strength and is resistant to cold flow when formulated or contacted with drugs, co-solvents, excipients or skin penetration enhancers.